As multimedia information processing systems increase in popularity, system designers must consider new techniques for controlling the processing and display of data simultaneously generated by multiple sources. In particular, there has been substantial demand for processing systems which have the capability of concurrently displaying both video and graphics data on a single display screen. The development of such systems presents a number of design challenges, not only because the format differences between graphics and video data must be accounted for, but also because of end user driven requirements that these systems allow for flexible manipulation of the data on the display screen.
One particular technique for simultaneously displaying video and graphics data on a single display screen involves the generation of "windows." In this case, a stream of data from a selected source is used to generate a display within a particular region or "window" of the display screen to the exclusion of any non-selected data streams defining a display or part of a display corresponding to the same region of the screen. The selected data stream generating the display window "overlays" or "occludes" the data from the nonselected data streams which lie "behind" the displayed data. In one instance, the overall content and appearance of the display screen is defined by graphics data and one or more "video windows" generated by data from a video source occlude a corresponding region of that graphics data. In other instances, a video display or window may be occluded or overlaid by graphics data or even another video window.
In the multimedia environment, the "windowing" described above yields substantial advantages. Among other things, the user can typically change the size and location on the display screen of a given window to flexibly manipulate the content and appearance of the data being displayed. For example, in the case of combined graphics and video, the user can advantageously create custom composite visual displays by combining multiple video and graphics data streams in windowing environment.
In order to efficiently control windows in a multimedia environment efficient frame buffer management is required. Specifically, a frame buffer control scheme must be developed which allows for the efficient storage and retrieval of multiple types of data, such as video data and graphics data. To be cost competitive as well as functionally efficient, such a scheme should minimize the number of memory devices and the amount of control circuitry required and should insure that data flow to the display is subjected to minimal delay notwithstanding data type.
One of the major difficulties in managing video in a combined video and graphics windowing environment results from the fact that the video data being received and displayed are constantly being updated, typically at a rate of thirty frames per second. In contrast, the graphics data are normally generated once to define the graphics display and then remain static until the system CPU change that graphics display. Thus, the occlusion (overlay) of video data with graphics data requires that the static graphics data "in front of" the video data not be destroyed each time the video window is updated. A second concern with windowing systems operating on both video and graphics data is the formatting differences between the video and graphics data themselves since video is typically digitized into a YUV color space while graphics is digitized into an RGB color space. Hence, any combination video and graphics windowing system must have the capability of efficiently handling data within both the YUV and RGB formats.
Thus, due to the advantages of windowing, the need has arisen for efficient and cost effective windowing control circuitry. Such windowing circuitry should allow for the simultaneous processing of data received from multiple sources and in multiple formats. In particular, such windowing control circuitry should be capable of efficiently and inexpensively controlling the occlusion and/or overlay of video and graphics data in a windowing environment.